OBJECTIVE AND IMPORTANCE: The expected time interval for resolution of hydromyelia after Chiari I decompression is lacking in the literature. This case report highlights one instance of delayed resolution of Chiari-induced hydromyelia. CLINICAL PRESENTATION: We report an adolescent girl with a Chiari I malformation and hydromyelia. INTERVENTION: A suboccipital craniectomy and C1 laminectomy with intradural exploration and duraplasty were performed. Serial imaging at 1 and 2 years after posterior cranial fossa decompression with duraplasty demonstrated no change in the size of the hydromyelia. At 3 years after surgery and before reoperation for continued hydromyelia, repeat magnetic resonance imaging demonstrated significant diminution of the fluid cavity. If this hydromyelia did resolve as a result of surgery, the interval for radiological observation clearly needs to be reconsidered. CONCLUSION: Unfortunately, this is an area in which the literature is lacking. With this case as a nidus, studies are now necessary to determine the range of time necessary for Chiari I malformation-related hydromyelia to resolve.